I think it's a little column A and a little column B. They outpitched their peripherals, likely in part, to their excellent defense. In the playoffs, you really only need four starters (though teams have proven recently you can do with less, sometimes two or three). They also got more from Cade Horton than anyone expected, and had he not fractured a rib a week before the seasons end, I think the Cubs SP's numbers would be generally fine for now.
They could have also overpaid and gotten one more guy, or had bought one more impact reliever.